It has recently been shown that the radiation outputs which were previously still inadequate for semiconductor lithography in the extreme ultraviolet spectral region around 13 nm can obviously be substantially further increased only by means of more efficient emitter substances such as tin or lithium or combinations thereof (DE 102 19 173 A1). DE 102 19 173 A1 also already makes reference to the technical problem that very high temperatures of the discharge source are required for evaporation when using metal emitters, and condensation of the metal vapors in the interior of the sources must be prevented because otherwise operational failure can be expected within a short time. With an electric discharge, another problem arises in that the electrodes take on such high temperatures due to the high currents and the direct proximity to the plasma that the high-melting electrode material (e.g., tungsten) is loaded near the melting point, and other emitter material which is not provided directly in the excitation site for plasma generation is also evaporated and generates an unwanted debris component.
When tin is supplied in the form of gaseous tin compounds, e.g., SnCl4, there is the added disadvantage that more emitter material is introduced into the discharge chamber than is necessary for the EUV emission process. As a result of condensation, these residual amounts lead to deposits of tin layers and—when using SnCl4—chlorides which cause the source to fail after a relatively short operating period.
An important approach to a solution for preventing excess emitter material in the plasma chamber of the pulsed radiation source is to make available for every pulse only as much emitter material as can be completely converted into radiating plasma through the input of energy (through electric discharge (GDP), laser beam (LPP) or electron beam) for generating the plasma at the excitation site.
When metal emitter material is supplied as a regular series of liquid droplets which is generated through a nozzle under a certain pressure or is directed to the electrodes for coating, the nozzle must be connected to a reservoir of liquid emitter material. In so doing, it is necessary to interrupt the supply of emitter material to the nozzle in order to fill the reservoir because the pressure level for the constant generation of droplets does not remain the same while the reservoir is being filled.
While stationary electrodes reach a surface temperature above the melting temperature of the electrode material itself (3650 K for tungsten in any case) after a few pulses at repetition rates in the kilohertz range, an equilibrium temperature can be kept low enough by rotating the electrode that even the temperature peaks on the electrode surface remain appreciably below the melting temperature of tungsten. The temperature peaks are still far above the melting temperature of the emitter material (505 K for tin) so that, in addition to the controlled laser evaporation, there is an uncontrolled deposition of tin on the electrodes.
To prevent uncontrolled evaporation of emitter material, US 2007/0085044A1 discloses a device for generating extreme ultraviolet radiation with rotating electrodes in which an injection device injects a series of individual volumes of emitter material in a discharge area of the rotating electrodes at a defined distance from the latter. An energy beam is directed to the site in the discharge area where the individual volumes arrive so as to be synchronized with the frequency of the gas discharge for the plasma generation so that they are successively pre-ionized by the energy beam. To this end, the injection device is designed in such a way that the individual volumes are supplied at a repetition frequency that is adapted to the frequency of the gas discharge. However, this has the drawback that no measures are provided for ensuring constant droplet generation.
For stable generation of radiation from a series of droplets (low pulse-to-pulse fluctuations and no outages), every droplet must be supplied at the desired repetition rate at a location at a distance (typically 50-1000 mm) from the nozzle. This demands a very stable generation of droplets, i.e., constant droplet size, flight direction, and droplet velocity. This necessarily calls particularly for a very constant, regulated pressure of the emitter material in the droplet generator (in the nozzle).
The adjustment of a suitable pressure for a liquid emitter material can be carried out by applying a pressure gas to the liquid as is described, e.g., in U.S. Pat. No. 7,122,816 B2. In particular, a determined pressure is maintained in the droplet generator and also in an emitter material reservoir. A connection line with a controllable valve is provided between the two vessels so that the droplet generator can be refilled during the course of operation and so that a controlled melting of solid emitter material in the emitter material reservoir, depending on the amount of emitter material delivered to the droplet generator, can always be kept on hand at the same time and to make refilling possible also while the droplet generator is operating.
However, the commercially available pressure regulator used for this purpose to adjust and regulate a defined gas pressure does not solve the problem which arises when gas at elevated pressure is applied to a liquid metal emitter material such as tin due to the solubility of the gas (mixing of the liquid) in the liquid metal.